Statistical physics in the twentieth century was dominated by the study of systems at or near thermal equilibrium, but the past two decades have seen an increased focus on far-from-equilibrium behavior. This shift has been driven by many factors, including breakthroughs in basic theory; novel experimental tools for manipulating nanoscale objects; insights from dynamical systems and computer simulations; and a growing appreciation that the molecular processes that sustain life occur away from equilibrium. Non-equilibrium dynamics play a crucial role in the growth and response of novel materials, as well as in the behavior of systems at astrophysical length scales, such as plasmas and in the formation of high-energy cosmic rays. The field of active matter has recently emerged as an exciting area of non-equilibrium statistical physics. Active matter refers to systems containing many individual units or "agents," such as molecular motors attached to cytoskeleton cells in a sample of living tissue or schooling fish, each of which directionally moves under the power of an energy source. The collective behavior of active matter emerges from the non-equilibrium dynamics of the agents.